I had to purchase the Logitech Bolt receiver to connect Logitech’s MX Keys for Mac to my iMac. In a similar way, I had to use the Unifying receiver, Logitech’s previous generation Bluetooth receiver, to connect my mouse. This is because my Mac, being old, doesn’t support the flavour of Bluetooth that these peripherals do. However, the mouse came with a USB receiver in the box whereas the keyboard didn’t. Several months later, because Logitech were constantly out of stock, I managed to get a Logi Bolt receiver from a third party. Problem now is that with this type of receiver, you have to download an app to connect it - and guess what? Yep, there isn’t a version of the app for the operating system my iMac runs.
MX Keys keyboard, Logi Bolt USB Bluetooth receiver - both useless to me. I should have stuck to an Apple keyboard and mouse and I wholeheartedly suggest you do the same. Bad concept, poorly executed.